1. Field of the Invention
The present invention relates to a honeycomb structure.
2. Discussion of the Background
In recent years, particulates (hereinafter, also referred to as “PM”) such as soot contained in exhaust gases discharged from internal combustion engines of vehicles such as buses and trucks, construction machines and the like have raised serious problems as contaminants harmful to the environment and the human body. For this reason, various honeycomb structures made of porous ceramics have been proposed as honeycomb filters for collecting PM in exhaust gases to purify the exhaust gases and as catalyst carriers allowing exhaust gases pass through the inside thereof to convert the contaminants in the exhaust gases.
Conventionally-known honeycomb structures as above described include a honeycomb structure having a honeycomb block including a combination of a plurality of honeycomb fired bodies each having a large number of cells. FIG. 1A and FIG. 1B schematically illustrate examples of honeycomb fired bodies positioned at the outermost periphery of the honeycomb structure, among honeycomb fired bodies used to manufacture a conventional honeycomb structure (JP-A 2004-154718). Honeycomb fired bodies 1410 and 1420 illustrated in FIG. 1A and FIG. 1B respectively have cells 1411 and 1421 positioned closest to the curved surface forming the peripheral face of the honeycomb block. The cells 1411 and 1421 respectively have a substantially-triangular shape and a substantially-trapezoidal shape. Each of the cells 1411 and 1421 has a side formed along the curved surface.
It is necessary to seal either ends of the cells in the honeycomb fired body to use a honeycomb structure including the honeycomb fired body as an exhaust-gas purifying filter. The cells 1411 and 1421 positioned at the outermost periphery of the honeycomb fired bodies 1410 and 1420 have small opening areas, and therefore, a plug material paste may not easily fill the cells or may easily overflow or protrude from the cells. As a result, the cell is not sufficiently sealed.
Use of a honeycomb structure including the honeycomb fired body with insufficiently-filled cells as an exhaust-gas purifying filter causes a problem that the honeycomb structure allows exhaust gases flowing therein to flow out from the same cell without passing through a cell wall, resulting in a failure to fulfill the function as a filter.
There has been proposed another honeycomb structure in which a cell positioned at the outermost periphery and a cell not positioned at the outermost periphery have the same shapes so as to facilitate the filling of the cells with the plug material paste (WO 2008/126335 A1). FIG. 2A and FIG. 2B respectively illustrate exemplary conventional honeycomb fired bodies in which a cell positioned at the outermost periphery and a cell not positioned at the outermost periphery have the same shapes. All of cells 1111 and 1121 in honeycomb fired bodies 1110 and 1120 have substantially-square shapes in cross sections perpendicular to the longitudinal directions thereof. Positions of the cells 1111 and 1121 are designed so that the cells 1111 and 1121 are placed at regular intervals. In addition, peripheral walls 1116 and 1126 have level differences (difference in level such as stairs) corresponding to the positions of the cells 1111 and 1121 positioned at the outermost periphery so that a cell positioned at the outermost periphery and a cell not positioned at the outermost periphery have the same shapes.
According to this conventional honeycomb structure, even a cell positioned at the outermost periphery, which conventionally has a smaller opening area and is not easily filled with a plug material paste, has the same opening area as that of a cell not positioned at the outermost periphery. Accordingly, the cells are easily filled with a plug material paste so that manufacturing efficiency of the honeycomb structure is improved to some extent.
The contents of JP-A 2004-154718 and WO 2008/126335 A1 are incorporated herein by reference in their entirety.